In most livestock species, it is well documented that intact males out perform, are more efficient and are superior in most carcass traits compared to their castrated counterparts. In swine, Wood and Riley (1982) and Hines et al. (1969) found that boars grew faster than barrows and were more efficient in converting feed to tissue. Additionally, Kuhlers et al. (1976) reported that boars had significantly less backfat and a larger loin eye area than barrows of the same live weight. For this reason in some European countries (e.g., Great Britain, Ireland and Spain) most slaughter pigs are intact males (Walstra, 1984). Yet, with the obvious advantages associated with the production of boars, come the problems that encouraged the development of castration.
The most economically important problem with growing boars for meat production is the unpleasant odor or "boar taint" found in boar meat during cooking. Studies suggest that between 5 and 35 percent of market weight boar carcasses are tainted at a level offensive to consumers (Malmfors and Lundstrom, 1983). Additional concerns with intact male production are increased aggressive behavior, increased rates of down pigs, demands on facilities, separate sex feeding and increased danger to human caretakers.
As discussed previously, the major economic shortcoming of intact male swine production for slaughter is boar taint. This unpleasant odor was first studied by Lerche (1936) who showed the early appearance of the odor coincided with the onset of puberty and disappeared following castration. Patterson (1968) using chromatography-mass-spectrometry isolated 5-alpha-androst-16-en-3-one (androstenone) as a major component of this odor. Shortly afterwards this finding was confirmed and 5-alpha-androst-16-en-3-alpha-ol and 5-alpha-androst-16-en-3-beta-ol were identified as minor contributors (Beery and Sink, 1971; Berry et al., 1971). About the same time another much different compound skatole, which possesses a strong fecal odor, was found to act synergistically with and strengthen the foul odor of androstenone (Vold, 1970; Walstra and Maarse, 1970).
Skatole and androstenone are very unrelated in their production. Androstenone, and other 16 androstenes that are found bound in boar fat, are produced in the testes with pregnenolone as the precursor (Brooks and Pearson, 1989). These compounds are released into the blood via the spermatic vein and are stored in fat (Bonneau and Terqui, 1982). The 5-alpha-androst-16-en-3-one configuration is the predominant storage form because it is the most lipophilic and it is therefore the major component responsible for boar taint (Claus, 1979). During sexual stimulation these steroids can be released back into the bloodstream to travel to the submaxillary gland for use as an important pheromone in saliva (Gower, 1972; Claus, 1979). It is apparent that this storage procedure is also reversible over time because after castration of older males, steroid concentrations in adipose tissue decline (Claus, 1976).
The levels of androstenone found in boar fat vary widely and are affected by age, weight, genetics and stage of sexual maturity (Jonsson and Andresen, 1979; Bonneau, 1981; Willeke, 1980; Walstra, 1984). Additionally, they may be sensitive to external stimuli such as rearing environment or sexual exposure although the literature is somewhat conflicting. As a general rule androstenone production begins increasing slowly when the animal reaches about 70 kg or approximately 4 months of age (Bonneau, 1981). It is also known that the highest incidence of objectionable odor in fat occurs in boars weighing over 95 kg and greater than 5 months of age (Walstra, 1984).
The other component isolated as a major factor in boar taint is skatole. It is produced by lactobacilli in the hindgut of the pig as these microorganisms break down tryptophan (Yokoyama and Carlson, 1979). Skatole concentration can be altered somewhat by diet and is normally found in excess of threshold levels (0.20 ppm) in boars. However, these levels are rarely reached in barrow or gilt fat (Mortensen et al., 1986) even though gut microflora differs only slightly between the sexes. This may indicate a hormonal influence either in the absorption rate of skatole or the rate of storage and degradation once absorbed (Lundstrom et al., 1988).
Moderately high to high correlations have been reported between sensory panel scores for boar odor and laboratory analysis for both androstenone and skatole levels in boar fat. Lundstrom et al. (1988) reported a correlation of r=0.53 for androstenone concentration compared with sensory panel scores for boar odor and a slightly higher correlation (0.65) for skatole level compared to the same sensory scores. Additionally, a smaller (0.32) correlation was reported between skatole and androstenone concentrations. Hansson et al. (1980) reported a coefficient of determination of 36 percent for boar odor by using androstenone alone; however, when skatole values were added to the model an r-squared value of 0.5 was achieved. Undoubtedly both compounds are important in determining boar odor levels and their interaction is very likely.
Historically, surgical castration has been the choice of most swine producers to deal with boar taint. Though relatively easy, inexpensive and very effective it is not the perfect solution. The testes of a pig is highly vascular and filled with nerves and for these reasons pigs are usually castrated at an early age and knife castration is not done on mature boars. Hence besides reducing growth and carcass quality, there are other risks involved in surgical castration: infection, severe herniation and killing or stunting animals due to large losses of blood during the operation or after re-injury by other animals.
Recently other methods have been tried to overcome the problems associated with surgical castration. For example, intact males have been treated with progestagen either through an implant or in the diet (Berger et al., 1981; Kluber et al., 1988). Immunization of boars against one of the compounds responsible for boar taint has also been tried. (Williamson et al., 1985; Brooks et al., 1986). Both of these methods have serious problems: hormonal treatment must be repeated several times and is not acceptable in swine for human consumption in some cultures. Immunization is not uniformly effective in all animals and there is some risk that cross-reacting antibodies will be induced causing undesired side effects.
In view of the problems with surgical, hormonal and immunocastration, there is a continuing need for an improved method of castration that controls boar taint, preferably without affecting growth and carcass quality. The present invention relates to a method of chemical castration and accomplishes those goals.
By way of review, there are two approaches to chemical castration, the first being to inject a sclerotic agent into the vas deferens of an intact male causing an occlusion in the duct blocking the transport of sperm and rendering the animal infertile. This type of chemical sterilization induces infertility but does not reduce the production of testosterone or other testicular steroids responsible for boar taint and male aggressiveness. Many compounds have been shown effective for this purpose: 10 percent silver nitrate or 3 percent formalin (Pineda et al., 1976); 95 percent ethanol (Freeman and Coffee, 1973); dondren (Bierschwal and Ebert, 1961); and quinacrine (Malaviya et al., 1974).
The second type of chemical castration and the type employed in the present invention involves an agent responsible for tissue modification and subsequent disruption of testicular function. The particular effect observed depends on the chemical and concentration injected, species, maturity of the animal and so forth. U.S. Pat. Nos. 4,156,427 and 4,339,438 to Fahim describe the use of zinc tannate as a chemical sterilant. Applicant's early data were collected on sexually mature Holzman strain rats injected with zinc tannate. Zinc tannate was also tried on cattle but caused difficulty in walking. The extent of discomfort in movement did not appear to be dose dependent and went away within 2-3 days after injection. No loss of appetite or reduction in feed consumption was reported. With pigs, however, and particularly with baby pigs, applicant has found that zinc tannate is neurotoxic when injected into the testes causing convulsions, loss of coordination and sometimes death.
Because of the difficulty in walking problem associated with the injection of zinc tannate, lactic acid and other acidic compounds, applicant developed a neutral chemical sterilant that can be injected without causing discomfort. U.S. Pat. No. 5,070,080 to Fahim describes the use of a neutralized solution of a mineral gluconate and amino acid, a typical example of which is zinc arginate. When applicant injected zinc arginate in pigs (see Example 1 below), the boars were sterilized but boar taint was not reduced. Applicant then evaluated other zinc salts, the present invention relating to the discovery that mineral acetate salts are not neurotoxic and, if administered in a proper amount, chemically castrate pigs in a manner that eliminates boar taint without reducing growth and carcass quality.